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  CHAPTER (III) PROPERTIES OF PURE SUBSTANCES   3-1: Pure substance: It is a substance that has a fixed chemical composition. As water, nitrogen, helium, and carbon dioxide for example. Also, a mixture  of various chemical elements or compounds are considered as a pure substance as long as the mixture is homogeneous. As air for example. But a mixture of oil and water is not a pure substance. A mixture of two or more phases  of a pure substance is still a pure substance as long as the chemical composition of all phases is the same. As a mixture of ice and liquid water, for example. 1  Note that A mixture of liquid air and gaseous air is not a pure substance, this is due to different components in air condensing at different temperatures at specified pressure. A pure substance exist in different phases. At room temperature and pressure, copper is a solid, mercury is a liquid, and nitrogen is a gas, for example. 3-2:Phase Change Processes of Pure Substance: Attention in this section is focused on the liquid and vapor phase   and their mixture. Water  is used to demonstrate the basic principles involved, and all the pure substances exhibit the same general behavior. During the heating process of water to change its phase from liquid to vapor at sea level i.e; at atmospheric pressure, we will recognize the following states, as shown in (Figure 3-1):   2  Fig.3-1   3  1)The compressed liquid (or sub-cooled liquid): It is the states of liquid where it is not about to vaporize. (all liquid states up to just before state 2, as state 1). 2) The saturated liquid: It is the state of liquid where it is about to vaporize. (state 2 exactly). 3) The saturated vapor: It is the state of vapor where it is about to condense. (state 4 exactly). 4)The saturated liquid-vapor mixture: It is the states where the liquid and vapor phases coexist in equilibrium. (states above state 2 and before state 4, as state 3). 5) Superheated vapor: It is the state of vapor where it is not about to condense. (states above state 4, as state 5). 4
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